This invention relates generally to optical devices, and more particularly, to an optical switch for selectively aligning or misaligning the optical signal of any one of several input optical waveguides with any one of several output optical waveguides. The invention is in the field of blocking optical switching and optical attenuating devices.
Numerous optical switches have been developed for selectively switching an optical signal from one waveguide across an interface to another waveguide. Regardless of design, insertion loss and reflectance remain central to judging the optical performance of a switch. These parameters combine to describe the efficiency with which the switch passes light. The relative insertion loss and the reflectance performance of an optical switch are determined by physical elements of the switch design.
Insertion loss is a measure of how much light the switch blocks between the input and output waveguides. Higher insertion loss means more light is blocked by the switch. An ideal switch has minimal insertion loss. The insertion loss of an optical switch is determined primarily by the ability of the switch to precisely align the input waveguide with respect to the output waveguide. Offsets in longitudinal, angular, and transverse alignment must all be controlled to minimize insertion loss. Historically, transverse offset has been the most difficult source of insertion loss to control in mechanical optical switch designs.
Reflectance is the ratio of reflected light power to incident light power in an optical waveguide. Fresnel reflections at the discontinuity between the input and output waveguides are the prime source of reflectance in an optical switch. Higher reflectance adversely affects performance of an optical system in two ways. First, transmitted optical power is decreased as more light is reflected. Second, the reflected light is often transmitted back to the laser source. There it creates a proportional amount of noise on the signal. Several methods are available to reduce the reflectance such as index matching mediums, anti-reflective coatings and providing an angled endface on the waveguides. The angled endface gives the best reflectance performance.
Existing optical switches can be categorized by their mode of operation as either electronic or mechanical switches. Electronic switches have no moving parts and divert light with electrical or acoustic energy. Mechanical switches physically move optical elements to perform the switching function.
Many prior art mechanical optical switches exist for comparison to the present invention of a two-beam optical switch. Discussion of the prior art is based on two relevant physical characteristics, degrees of freedom and adjustability. Degrees of freedom refers to the number of kinematic degrees of freedom available for positioning waveguides. Adjustability describes the available motion control within each degree of freedom. A switch with adjustability can attain and hold several intermediate positions within each degree of freedom.
Most available mechanical optical switches offer a single degree of freedom. Many of these are rotary designs wherein the waveguides on both input and output sides of the switch are coaxially supported for rotation relative to each other around a single axis of rotation. U.S. Pat. Nos. 5,317,659 to Lee (1994), 5,420,946 to Tsai (1995), and 4,378,144 to Duck et al. (1983) are all similar designs with both adjustability and one degree of freedom. The Duck et al. patent describes a widely used optical switch that is representative of the other rotary designs, and displays many inherent disadvantages of the rotary designs.
This prior art rotary devices arrange output optical fibers in a circle. The input fiber is placed on an arm that is attached to an actuator such as a stepper motor. A beam expander lens is attached to each input and output fiber. The input fiber actuator is coaxial with the circle defined by the output fiber lenses. The input fiber lens faces the output fiber lenses. The actuator rotates to align the input fiber lens to any of the output fiber lenses. Adjustability is provided by the use of a stepper motor to actuate the arm.
Several single degree of freedom switches also exist that are not rotary designs. They depend on beams or arms to move the light signal between fibers. U.S. Pat. No. 5,078,514 to Valette (1992) and U.S. Pat. No. 5,024,500 to Stanley (1991) both offer one degree of freedom and adjustability. Both show a single switched fiber mounted along the length of a beam. When the beam is actuated, it bends to align the single fiber with any of the remaining fibers mounted to the base.
Non adjustable single degree of freedom mechanical switches are shown in U.S. Pat. No. 4,946,247, to Muska et al. (1990), U.S. Pat. No. 5,239,599 to Harman (1993) and U.S. Pat. No. 4,146,856 to Jaeschke (1979).
Several two degree of freedom mechanical switches do not offer adjustability. Adjustability is often removed to reduce switch size for design applications that require a limited number of fibers, such as the device shown in U.S. Pat. No. 4,220,396 to Antell (1980).
The best optical performance is delivered by optical switches with two degrees of freedom and adjustability. U.S. Pat. No. 4,886,335 to Yanagawa (1989) and U.S. Pat. No. 5,438,638 to Anderson (1995) both offer optical switches with two degrees of freedom with adjustability. The Yanagawa patent combines two linear stages to form a very large and complex optical switch. The Anderson switch is smaller and has two degrees of freedom with full adjustability available for positioning fibers.
However, the Anderson ""638 patent has disadvantages.
The input and output fiber endfaces of the Anderson switch cannot be angled to incorporate the preferred angled endface to improve reflectance performance. The ferrules that hold fibers in Anderson must rotate up to 180 degrees to couple a fiber pair and this could move the endface angles as much as 180 degrees out of phase, causing a mismatch and thus insertion losses.
The Anderson design also has a limited fiber capacity. As the fiber count increases, more fibers are added to each bundle thereby increasing the bundle radius. The angular resolution required to position the outermost fibers decreases as the bundle radius increases. The result is a steady degradation in the repeatability of insertion loss and an increase in the switching time for each pairing as capacity increases.
The Anderson design will exhibit drift and signal interruption from wear. Each switching cycle generates a high degree of relative motion between the ferrules and their V-grooves. This motion creates particles that can interrupt the signal. It also wears the ferrules down steadily decreasing their diameter and creating signal drift.
The Anderson design requires a lengthy fiber search process. Since there are two locations for each possible waveguide pairings and no simple way to predict the location of the one pairing given that of the other pairing, the algorithm used may waste time finding both pairings. Also, the search process can require human intervention to achieve optimization.
The Anderson patent construction is such that fibers near the center of the input bundle will not be able to couple to fibers near the center in the output bundle. The result is that the innermost fibers in both bundles go unused and, thus, their signal-carrying capacity is wasted.
The two-beam optical switch of the present invention overcomes difficulties described above and affords other features and advantages heretofore not available.
The present invention is an optical switching device consisting of two main waveguide mounting beams or arms. The first beam includes one or more input waveguides and the second beam includes one or more output waveguides. The waveguides preferably are arranged in bundles. The beams are arranged such that the waveguide bundles carried by the respective beams face each other and each beam has a separate axis of rotation. The switch is capable of switching any of the input waveguides to align with any of the output waveguides. Waveguide interconnections are accomplished by moving the beams to adjust the location of waveguides carried by each beam to bring the desired input and output waveguide pair into close optical alignment. The driving motion for each beam is deamplified to provide the precision needed to accomplish accurate alignment, or misalignment if attenuation of the signal is desired. An optimum position is determined for all possible coupling waveguide pairs. These positions are specific to each individual switch and are pre-programmed into a device memory forming part of a switch control at the time of manufacture.
In operation, the adjustability of the present device can also be used to purposefully misalign any selected waveguide pair so as to attenuate the switched signal. Alternatively, the switch can be constructed without adjustability for applications that require limited switched waveguide capacity. In the non adjustable configuration, preferably electromagnetic actuators are used to drive the beams between two adjustable end stops, or the beams may be spring loaded against one stop.
The present invention displays excellent positioning resolution. In one aspect of the invention the beams are pivoted at a point and carry the waveguides at a location spaced from the pivot. Movement of the beams is actuated with commonly available stepper motors and reduction gear drives in the adjustable configuration. In both adjustable and non adjustable forms, motion of the beam is preferably actuated at a point on each beam so that the distance from the actuation point to the pivot point is greater than the distance from the pivot point to the waveguide endface. Because motion is larger at the actuation point, the error margin for setting the end stops or locating the optimal fiber alignment position is increased over other designs, and transverse offset errors are reduced.
Since the driving motion is de-amplified, the effects of wear at the driving motor and gear interface are reduced by the same ratio, resulting in long switch life. Wear that occurs at the rotational pins or end stops is minimal because so little relative motion is required to actuate the switch. The potential for signal interruption due to wear particles is thus reduced with this design. The distance between the wear point and the waveguide endfaces is large and there is no index matching medium to carry particles to the fiber endfaces. As a result, wear that does occur during operation does not impact optical performance. Finally, since the waveguide endfaces are not in contact during operation, there is no remaining source of wear particles near the waveguide endfaces to disrupt the optical signal.
In its adjustable configuration, the switch of the present invention is able to actively compensate for wear. Two of the input waveguides are attached to a light source and two of the cutput waveguides are attached to a light detector. The input and output waveguides used are located at diagonally opposite corners of their respective waveguide matrices. The drive algorithm makes use of these waveguides to establish a zero or home position for each beam. All other waveguide locations are referenced to these home positions. The same routine is used to periodically realign the matrix and compensate the device for any wear that may have occurred. This is done by re-optimizing each of the opposing locations at power-up or whenever desired.
The present invention has a lower cost, both in material and manufacturing. The few simple steps required to assemble the parts into a completed switch do not require precision manufacturing and result in improved optical performance. The waveguide assemblies are potted together in ferrules and can be tested before assembly into the switch. The absence of expanded beam lenses leads to lower insertion loss and lower material cost. The negligible amount of waveguide bundle rotation needed to achieve all desired waveguide interconnections allows the waveguide endfaces to be polished at an angle for superior reflectance performance.
The search algorithm for the switch is simple. There is a single optimum beam position for each waveguide pairing. Since each waveguide carried by a beam moves roughly the same amount for a given actuation distance, the search routine can operate at a relatively high speed to quickly find all optimal fiber pairings. The positions of optimal waveguide pairing correspond to the matrix arrangement of the waveguides. Given the exact locations of two waveguide pairings on the corners of the matrix (home positions), the routine uses data on the physical size of the fiber to predict the approximate locations of the remaining waveguide pairings. These design advantages further help to decrease search time and make it more cost effective.
The present invention makes efficient use of space and can be readily adapted to suit a wide variety of design needs. A smaller package size is achieved for the same switched waveguide capacity. This is because two degrees of freedom allows the input and output waveguides to be arranged in more efficient matrices rather than in lines or circles. Additionally, all possible interconnections from each input waveguide to each output waveguide are useable or reachable. Optimizing design parameters such as beam length, deamplification ratio, gear ratio of the drive, and the like for the particular size constraints or performance needs of the application is easily done.
The present invention provides an optical switch having low insertion loss and low reflectance while providing performance that is stable and consistent over time, temperature and humidity fluctuations, cycling and. vibration. The optical switch of the present invention is made of relatively inexpensive components and is simple to manufacture. The optical switch is capable of switching from any number of input waveguides to any number of output waveguides, while remaining compact and adaptable to meet a wide variety of design objectives.
Other advantages of the invention will become apparent from the following detailed description and from the appended drawings in which like numbers have been used to describe like parts throughout the several views.